For research purposes, Henry Wiencek paid a visit to George Washington's Mount Vernon and immersed himself in that tourist attraction's re-enactment of slavery. The hands-on farm supervisor soon had him thumping at a pile of straw with a flail. ''This was one method of threshing,'' writes Mr. Wiencek (whose earlier books include ''Mansions of the Virginia Gentry'' and ''Plantations of the Old South''). ''Instantly, the tedium of the task became obvious -- the same motion in the same place in endless repetition. I could not imagine doing this from the early dawn to the late dusk of a summer day.''

In his revisionist new book about Washington, Mr. Wiencek examines the first president's own dawning comprehension of what slavery entailed. And the process of fathoming Washington's moral evolution is not a simple one. As the least candid of Founding Fathers (''then, as now, no one could quite tell how he made up his mind about something,'' Mr. Wiencek writes), he did not leave the kinds of diaries or letters that have made John Adams, Thomas Jefferson or Benjamin Franklin such hits as historical subjects. But Mr. Wiencek, threshing and all, rises to the challenge of turning Washington's very furtiveness into a source of fascination.

''An Imperfect God: George Washington, His Slaves and the Creation of America'' is not a book packaged with high praise from other historians. Instead, a full page of text on the back cover is devoted solely to articulating Mr. Wiencek's complex overview. However lionized Washington was as a military leader, he was far more wishy-washy where slavery was concerned. His path to enlightenment regarding race was something other than a hero's journey.

In no other realm was he so susceptible to pressure, nor was he faced with such intractable resistance to change (not least of it from his wealthy, self-interested wife). In no other realm did he behave as badly. (When he needed dentures, he was not above trying to appropriate slaves' teeth.) His final redeeming gesture -- leaving a will that freed slaves -- cannot be seen as a simple, bold stroke. It makes sense only in the larger, richer context that Mr. Wiencek's book vividly creates.

''An Imperfect God'' follows Washington from a difficult childhood (his father's early death would prefigure his own) to a marriage that brought him great privilege and responsibility. ''He came of age, and learned to be a master, on a racial borderland where the definitions and boundaries of race were dangerously fluid,'' Mr. Wiencek writes. This book offers many glimpses into the ways in which intertwined black and white family histories revealed the monstrousness of slavery-sustaining laws.

By nature, Washington was sufficiently prim to have copied out a document entitled ''Rules of Civility & Decent Behaviour in Company and Conversation.'' (Among these rules: ''Talk not with Meat in your Mouth'' and ''Kill no Vermin as Fleas, lice ticks &cc in the Sight of Others''). He also kept a record entitled ''Where & How My Time is Spent.'' For a man of this temperament, any slaves' reluctance to hold themselves to similar standards was infuriating. As a younger man, Washington could complain about matters like ''the deception with respect to the Potatoes.'' He could also be party to a slave raffle, documented by Mr. Wiencek, in which children could be won as prizes.

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The breaking up of slave families for reasons of profit was, by Mr. Wiencek's account, the first outrage to penetrate Washington's self-interest. He traces Washington's first awareness of this to time he spent in Williamsburg, Va., witnessing slave auctions held in response to an owner's embezzling. ''In modern terms, it was as if the collapse of a Wall Street brokerage, due to the malfeasance of its officers, had led to the sale of the children of the cleaning staff to pay the debts of corporate vice presidents,'' Mr. Wiencek writes.

And as a leader of soldiers, Washington was acutely aware of the importance of black soldiers even as he waffled over the question of their eventual freedom. ''George Washington won the Revolutionary War with an army that was more integrated than any military force until the Vietnam War,'' Mr. Wiencek maintains. His book offers evidence that the role of black soldiers under Washington's command was under-reported simply because it was taken for granted.

Rather than a debunking account, ''An Imperfect God'' is one that measures the slow growth of Washington's willingness to change. The author, with a great interest in genealogical research, points to many instances in which the situations of Washington's own real and alleged family members (including West Ford, whose possible identity as Washington's illegitimate son is explored but rejected -- he may instead have been a nephew) could not help but provide impetus for change.

One of the most strange and compelling stories included here is that of Ona Judge, a slave who escaped the Washington household and brought out the dissembling side of the president who supposedly could not lie. When Washington resorted to illegal means to pursue this woman, he concocted the idea that she had been stolen away by a Frenchman. This notion, Mr. Wiencek reveals, may have come from a popular novel that Martha Washington read.

''An Imperfect God'' has its own imperfections. However painstaking Mr. Wiencek is in some regards, he can be imprecise and even self-contradictory about minor matters. (Did Washington meet the black poet Phillis Wheatley, or only correspond with her? The book acknowledges uncertainty, then uses the word ''met'' anyhow.) But about the bigger picture, this much is clear: Washington changed. And the force of his moral and intellectual progress can now be understood more deeply than ever before.